Valves for hydraulic brakes

ABSTRACT

Hydraulically-operable rotary friction brake comprising a retarding device incorporating hydraulically actuated operating members have a hydraulic circuit including a valve whereby in periods of operation a major portion of the hydraulic flow is directed through the hydraulically actuated operating members and in periods of non-actuation only a minor proportion of the hydraulic flow is directed through the operating members.

This Application is a Continuation in part of our copending U.S. Patentapplication Ser. No. 610,426, filed Sept. 4, 1975, now abandoned.

BACKGROUND TO THE INVENTION

This invention relates to a hydraulically-operated friction brake of thekind wherein the hydraulic liquid also provides the medium for removingthe heat generated during operation of the brake. The construction andoperation of one such friction brake is disclosed in our GermanOffenlegungschrift 2446669 and in the U.S. Patent Application Ser. No.510,126 directly corresponding thereto. Said U.S. Application is nowU.S. Pat. No. 3,924,712, which claims priority from the same eightBritish Patent Applications as does said German Offenlegungschrift.

In general terms, the friction brake of the aforesaid Offenlegungschriftcomprises (i) a casing, (ii) a rotary member mounted in the casing,(iii) a pressure-operable retarding device arranged and adapted to actupon the rotary member to generate retarding torque. (iv) a pump forcirculating hydraulic liquid, (v) two separate fluid circuits providedbetween the pump and the retarding device, one being an actuatingcircuit arranged and adapted to operate the retarding device, and (vi)valve means comprising (a) a valve member, (b) a first valve elememtsecured to said member, (c) a second valve element slidable on saidmember and urged away from the first valve element by a spring andconstituting pressure sensitive means arranged and adapted to controldistribution of flow of hydraulic liquid between the two circuits, and(d) means for equilibrating hydraulic pressure across the valve means.

More specifically, the brake just recited comprises a casing, a rotarymember housed therein and connectable to a vehicle propellor shaft andassociated therewith a pressure-operated retarding device (iii)comprising (a) annular friction plates mounted on the rotary member, (b)annular stator plates mounted on the casing and interleaved with thefriction plates, the stator and friction plates (collectively referredto hereafter as the `pack`) being relatively axially slidable under theaction of (c) an hydraulically-operable actuator in the form of apiston-and-cylinder device or bellows piston to bring the stator andfriction plates into contact and generate therebetween a retardingtorque on the rotatable member. A pump circulates hydraulic liquidthrough at least one hydraulic circuit which includes the pack and acooling means and which has a branch through which hydraulic liquid issupplied to operate the actuator piston when it is desired to operatethe retarding device. In the arrangement disclosed, the casing is filledwith hydraulic liquid, leaving only such space as is necessary toaccommodate the increase in the volume of the hydraulic liquid whichoccurs when the liquid absorbs heat during periods of actuation of theretarding device, and the hydraulic liquid is circulated through thepack even during periods of non-actuation of the retarding device aslong as the vehicle to which the brake if fitted is in motion. Thiscontinuous circulation of at least a major proportion of the hydraulicliquid through the pack causes a significant amount of `drag` withabsorption of power which could be used more efficiently elsewhere.

BRIEF DESCRIPTION OF THE INVENTION

It is an object of the present invention to reduce the aforementioneddrag by providing in the brake just described a hydraulic circuit whichincludes valve means arranged and adapted to control the flow ofhydraulic liquid through the pack so that (1) during periods ofactuation of the retarding device at least a major proportion of theflow is directed through the pack and (2) during periods ofnon-actuation of the retarding device not more than a minor proportionof said flow is directed through the pack.

BRIEF DESCRIPTION OF PREFERRED EMBODIMENTS

The invention is based on the concept that if the volume of hydraulicliquid contained in the friction brake at the maximum operatingtemperature thereof, is less than the total volume of the interior ofthe brake (casing plus cooling means etc.) the pack can be used, ineffect, as a kind of centrifugal pump operating so that, if thehydraulic supply circuit to the pack is partly occluded or even closed,the pack will tend to pump itself substantially empty of hydraulicliquid; conversely if the effect of the supply occlusion is removed ordecreased and the circuit is occluded at the outlet from the pack, thepack will be flooded with hydraulic liquid. The second of these actionsis effected when it is desired to actuate the retarding device; thevolume of hydraulic liquid in the brake at the maximum operatingtemperature is selected so that the pack will remain substantially freefrom hydraulic liquid during periods of non-actuation of the retardingdevice. A high pressure "bleed" from the actuator to a point down-streamof the pack may be necessary for safety reasons. It may not be necessaryto provide a separate pump to circulate the hydraulic liquid; the use ofsuch a pump is however, preferred at present. The free space left withinthe brake when it contains the selected volume of hydraulic liquid maycontain air or a relatively inert gas such as nitrogen, this gas beingcompressed when the hydraulic liquid undergoes thermal expansion. Theambient pressure within the brake casing is preferably less than 10p.s.i. and more preferably less than 5 p.s.i.

In one particularly preferred embodiment, provision is made to circulatethe hydraulic liquid by means of a pump through two circuits, one beingthe circuit for actuation of the retarding device as aforesaid, and theother/being an `idle` circuit for circulation of a major proportion ofthe hydraulic liquid during periods of non-actuation of the retardingdevice. The valve means is then arranged and adapted to controldistribution of the hydraulic liquid to permit (a) circulation ofhydraulic liquid in said one circuit in turn through the retardingdevice, cooling means and the pump, whilst simultaneously transmittingactuating pressure to the actuator during periods of actuation of theretarding device and (b) circulation of at least a major proportion ofthe hydraulic liquid through the said other circuit to the cooling meansand the pump simultaneously with not more than a minor proportion of thehydraulic liquid through the said one circuit during periods ofnon-actuation of the retarding device.

Preferred embodiments of the present invention will now be furtherdescribed by way of example with reference to the accompanying drawings.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating the hydraulic flow through the circuitsof a vehicle brake, in the non-actuated condition;

FIG. 2 is a diagram similar to that of FIG. 1, showing the flow in thecircuits when in the actuated condition;

FIG. 3 is a diagrammatic cross-section of valve means suitable for usein the brake of FIGS. 1 and 2;

FIG. 4 is a diagrammatic cross-section of a modified valve means for usein the brake of FIGS. 1 and 2;

FIG. 5 is a diagrammatic cross-section of another valve means;

FIG. 6 is a diagram illustrating the hydraulic flow through the circuitsof a vehicle brake using the valve means of FIG. 5, in the actuatedcondition;

FIG. 7 is a diagram similar to that of FIG. 6 but showing the flow inthe `off` non-actuated condition;

FIG. 8 is a diagrammatic cross-section of another embodiment of thevalve means;

FIG. 9 is a diagram illustrating the hydraulic flow through the circuitsof a vehicle brake using the valve means of FIG. 8, in the non-actuatedcondition;

FIG. 10 is a diagram corresponding to FIG. 9, but illustrating the flowin the actuated condition; and

FIG. 11 is a side elevational view, with parts broken away and shown insection, of a hydraulically-operable vehicle retarder with which thebrake of this invention is utilized.

FIGS. 1 and 2, in conjunction with FIG. 11, illustrate schematically thehyraulic liquid flow circuits and functioning of a friction brake of thekind set forth in our German Offenlegungschrift No. 2446669 and in theU.S. Pat. No. 3,924,712 directly corresponding thereto. Referringspecifically to this prior disclosure, a retarding device in the brakecomprises annular friction plates 1, axially slidable on and driven by arotatable shaft which is to be braked, interleaved with annular frictionplates 2 which are themselves axially slidably relative to said shaft onpins secured in the brake casing; the brake includes a pump 3, drivenfrom the rotatable shaft, a heat exchanger 4 for cooling the hydraulicliquid, a valve means 5 for controlling the distribution of flow ofhydraulic liquid within the hydraulic circuit and an actuator in theform of an annular bellows 6, all in fluid communication, exactly as inthe corresponding components of the brake disclosed in our aforesaidGerman Offenlegungschrift No. 2446669. The duct 7, linking the valvemeans 5 directly to the heat exchanger, is provided with a device 8 forventing to the interior of the casing hydraulic liquid in said duct 7,whenever the pressure of the liquid in the duct (and hence in the heatexchanger) reaches a predetermined limit.

In use, the brake contains a volume of hydraulic liquid insufficient tooccupy the whole of the free space within the casing at maximumoperating temperature and is mounted in a vehicle with the rotatableshaft connected to or forming part of the propellor shaft thereof; themotion of the vehicle rotates the shaft, driving the pump and causinghydraulic liquid to flow within the hydraulic circuits of the brake.FIG. 1 illustrates the situation when the vehicle is moving and thebrake is in the non-actuated condition, i.e. when no retarding torque isgenerated on the rotatable shaft. The continuous arrowed lines in FIG. 1illustrate the flow path of the major portion of the hydraulic liquid,from the pump 3 to the valve means 5, to the heat exchanger 4 and backto pump 3. The dotted line 9 illustrates the partly occluded path takenby a minor proportion of the hydraulic liquid, from the valve means 5through the pack of disengaged friction and stator plates to maintainthem `wet` and minimize friction between them, before rejoining the mainflow to the heat exchanger 4. In this case the feed 10 is vented toensure that there is not enough hydraulic pressure in it to operate theactuator (bellows piston 6). It will be appreciated that the brakecasing constitutes a reservoir for the liquid although it is notexplicitly shown in any of the figures.

FIG. 2 illustrates the situation when the valve means 5 is actuated toreduce the speed of the vehicle, for example on a downhill run; thevalve means 5 when thus actuated considerably alters the distribution ofthe flow. The line 10 is opened to allow hydraulic liquid to pressurisethe bellows piston 6 to compress the pack of friction and stator platesaxially of the shaft so that frictional engagement between said plateswithin the pack will generate a retarding torque on the shaft to slowits speed of rotation. Simultaneously, therewith the flow of hydraulicliquid is diverted through the pack, to the heat exchanger 4, back tothe pump 3 and valve means 5, (as shown by continuous arrowed lines) theduct 7 being closed. The circuit from the outlet of the pack isessentially occluded relative to the supply circuit to the pack.

FIG. 3 illustrates a preferred valve means, 5, for use in the circuit ofFIGS. 1 and 2; this comprises a body portion 11, constituted by aninsert within the brake casing and having a bore 12 therein with anenlarged diameter end portion 13. Mounted within said bore is anaxially-movable three element valve member in the form of a rod 14having first valve element 15 secured thereto, a second valve element 16slidable on the rod 14 and urged away from the element 15 by means of areturn spring 17, and a third valve elememt 18 having bores 19therethrough and held on the rod 14 by means of a split-ring 20. Duct 21leads to the outlet from the pump 3, of FIGS. 1 and 2; 22 is the outletfrom the valve means 5 to the heat exchanger 4 via duct 7; duct 23 isthe outlet to the line 9 feeding the pack 1, 2; duct 24 is the outlet tothe actuator feed line 10, and 25 is a by-pass line to permit flow ofhydraulic liquid from the pump outlet 21 to the duct 24 on downwardmovement of element 15 by the rod 14 and also to effect pressure balanceacross the valve means when the brake is in the actuated condition,thereby retarding the vehicle. Orifice 26 vents bore 12 to the interiorof the brake casing to prevent liquid being trapped therein. The endportion 13 is of enlarged diameter to permit flow of hydraulic liquidfrom the pump outlet 21 through axially directed bores 19 in element 18to outlet 23 when the rod 14 is moved downwards to actuate the brake.Otherwise the element 18 would prevent flow when displaced downward bythe rod 14. It should also be noted that the valve element 18 is made ina size such that hydraulic liquid may `leak` in a controlled mannerbetween it and the adjacent wall of the bore 12, to allow the flow of aminor proportion of the hydraulic liquid to the pack to keep itlubricated during periods of non-actuation of the retarding device. Thatis, the narrow space between the valve element 18 and the wall of thebore 12 acts as a controlled restriction on the hydraulic liquid flowthrough the pack.

The operation of the brake incorporating the valve means will be readilyapparent to those skilled in the art from the above description, takenin conjunction with the disclosures of our aforesaid GermanOffenlegungschrift and U.S. Pat. No. 3,924,712 corresponding thereto.

However, for the avoidance of doubt depression of the rod 14 against thereturn spring 17 progressively displaces the elements 15, 16 and 18downwardly to permit liquid actuation of the bellows piston 6 of FIGS. 1and 2, and thus engage the friction plates, whilst simultaneouslytherewith directing the liquid flow or at least a major portion thereofthrough the pack and heat exchanger in order to cool the brake.Releasing the rod 14 restores the valve elements to the position shownin FIG. 3 thereby disengaging the plates in the pack and cutting theliquid flow therethrough to the minimum necessary to merely lubricatethe pack.

FIG. 4 illustrates a modification of the valve means 5 as shown in FIG.3, primed numerals being used in FIG. 4 to indicate equivalent parts.

It can be seen that in this modification the valve means comprises avalve body 11' with bore 13' therein housing two valve elements 15' and16' mounted on actuating rod 14', with the outlets 21', 22', 23', 24'by-pass 25' and orifice 26' analogously disposed with respect to thecorresponding components of the valve means of FIG. 3. However, in thismodification, the third valve element 18' is located in a second bore 27in the valve body 11' communicating with outlet 23'. The element 18'includes a buffer element 29 dimensioned so as to define a narrowopening 31 between the valve element 18' and the body portion 11', whichopening communicates with a duct 30 (corresponding to duct 23 of FIG. 3)leading to the pack supply line 9. A spring 28 is provided to urge thebuffer element 29 into the position shown in FIG. 4, thereby partlyoccluding the duct 30.

In this modificaton, shown in FIG. 4 as also being in the non-actuatedcondition, hydraulic liquid leaks in minor proportion through outlet 23'and opening 31 to duct 30, thence to the pack so as to maintain a`trickle` feed to the `pack` during periods of non-actuation of theretarding device. When the valve rod 14' is moved downwards to actuatethe retarding device as described earlier, duct 22' is occluded, therebydirecting the flow into the duct 23' and against the underside, ofelement 18' which rises against the spring 28 to allow the full flow ofthe hydraulic liquid to the duct 30 leading to the pack and from thepack to the heat exchanger, outlet 22' now being closed by the element16'.

In the embodiment described above it may be found that a back-pressureis induced in the heat-exchanger 4, which tends to `flood` the pack inspite of the centrifugal pumping action of the pack described earlier.

In this case, resort may be had to the embodiment to be described withreference to FIGS. 5 to 7 inclusive, wherein components and partssimilar in function to those described above with reference to FIGS. 1to 4 are suffixed `b`.

FIG. 5 shows a valve means 5b in the non-actuated condition (the dottedlines showing the position of the valve element 16b when in the actuatedcondition, the general arrangement being similar in operation to thatshown in FIG. 3, except that the third valve element 18 is omitted andduct 26 is replaced by an inlet port 21b which is in direct fluidcommunication with the outlet from the heat exchanger and, when theretarding device is actuated (valve element 16b in the positionindicated by dotted lines), with the outlet 22b communicating with thepack and hence with the pump inlet. The outlet 23b communicates with theheat-exchanger inlet and is provided with a pressure relief device 35 inthe form of a sleeve 36 urged against a cup 37 by means of a spring 38(as described with reference to FIG. 3. in our German Offenlegungschrift2446669 and U.S. Pat. No. 3,924,712). The vent to the casing, 26b, isprovided with a similar pressure relief device 39 venting to the casingand operable at a lower pressure than the device 35 which is used toprevent excessive flow through the heat exchanger; (typically device 35may be arranged to vent at a pressure of 25 psi and device 39 at 15 psiby a proper selection of springs). The duct 22b, which in thisembodiment supplies line 9a (of FIG. 6) to the pack, is of sufficientwidth to define with the valve element 16b a narrow gap G when the valveis in the fully non-actuated position shown, so that the acutator feedduct 24b is vented via gap F to duct 22b, which is a point in the systemeffective to minimize the pressure on the actuator. In this embodiment,this is to the pack. The feed of a minor proportion of the totalhydraulic liquid flow to the pack is derived solely from leakage to duct22b around the valve element 15b and/or valve element 16b and throughnarrow gap G.

It will be seen by reference to FIG. 5 that when the retarder is in thenon-actuated conditon, very little of the hydraulic liquid can pass frominlet duct 26b to the outlet duct 22b (only the valve leakage referredto above) and the device 39 will `blow` to dump the liquid circulated bythe pump back into the reservoir space within the casing; hence theliquid simply flows back directly to the pump inlet. When the retarderis actuated, FIG. 6 applies; in this position, the pack accepts liquidfrom the duct 22b and hence the lower-pressure relief device 39 does not`blow` at normal operating pressure, although either it or the device 35may `blow` to dump liquid back into the casing if the pressure becomesexcessive. Operation of a retarder having a valve 5b is otherwisesimilar to that described above with reference to FIGS. 1 to 4.

In the valve means shown in FIG. 8 like parts bear like referencenumerals to those of the previous figures except for the use of thesuffix `a`, which is adopted to clarify the following description of theoperation of the valve means illustrated in FIGS. 9 and 10, since themode of operation is somewhat different from that of the valve meanspreviously described. In the embodiment of FIG. 8, the third valveelement 18 is omitted and the geometry of the valve is such that a gap50 is provided at the duct 22a when the valve elementt 15a is at itsuppermost position; the valve element 16a is biased against the circlip20a by two springs, one spring 33 extending the whole distance betweenthe elements 15a and 16a and one spring 34 (similar to spring 17 in FIG.3) being shorter and of greater compressive strength than spring 33 sothat the valve element 16a can ride up against the relatively lightspring 33 under pressure until it defines the gap 50 when the retardingdevice is in the non-actuated condition. This allows the liquid enteringat inlet duct 21a to take the path of least resistance back to thecasing. At the same time this creates just enough pressure to cause aminor proportion of the liquid to flow directly to the pack through theheat exchanger via duct 23a which has incorporated therein a pressurerelief device 41, as shown in FIGS. 9 and 10 when the retarding deviceis in the non-actuated condition, the flow through duct 23a beingrestricted as compared to the flow through the duct 22a in view of thefact that the duct 22a opens directly to the casing while the pack andthe heat exchanger as well as the associated piping presentsconsiderable resistance to flow. In this embodiment, duct 26a serves tovent surplus liquid from inside the bore 12a to the casing, therebyfacilitating operation of the valve means. Duct 24a feeds the actuatorbellows (6 in FIGS. 9 and 10), as before, inlet duct 21a correspondingto duct 21b of FIG. 5 and being connected to the pump 3. It will beappreciated from FIGS. 9 and 10 that the pack 1, 2 is fed directly fromthe heat exchanger through duct 23a and it is important to note thatoperation of control rod 14a cannot cause valve element 16a to occludethe duct 23a, axial projections 40 being provided to limit the downwardstravel of element 16a such that when in the fully actuated condition,the duct 21a still communicates with duct 23a to the heat exchanger 4.However, operation of the valve means and of the brake occurs in amanner broadly analagous to that described previously in relation to theother embodiments.

FIGS. 9 and 10 are, respectively, schematic non-actuated and actuatedflow diagrams for the valve means of FIG. 8. For convenience, all theducts bear exactly the same reference numerals as does FIG. 8. To avoidexcessive flow through the heat exchanger and pack a relief valve 41,venting to the casing (designated 42), is provided by way of the ventingdevice 8.

Insofar as the operation of the whole apparatus has previously beendescribed it is believed that FIGS. 8, 9 and 10 will be readilyunderstood by those skilled in the art as exemplifying only one way ofcarrying out the invention so as to reduce the flow through the pack toa controlled extent when the brake is in the non-actuated condition,thereby significantly reducing frictional drag.

In a brake having a pump rated at 15 gallons per minute per 1000revolutions per minute of the rotatable shaft, the minor proportion ofhydraulic liquid fed to the pack during periods of non-operation of theretarding device may be up to 5 gallons per minute but is preferablyfrom 2 to 3 gallons per minute.

In FIG. 11 there is illustrated a vehicle retarder which corresponds tothe retarder shown in FIG. 5 of U.S. Pat. No. 3,924,712, which retarderis a typical application of this invention. The retarder has annularfriction plates 1 axially slidable on and driven by a rotatable member102 which is to be braked. The friction plates 1 are inter-leaved withannular friction plates 2 which plates 2 are axially slidable relativeto the rotatable member 102 on pins 111 secured in a casing 112 of thevehicle retarder.

The vehicle retarder also includes a pump 3 and a heat exchanger (notshown) having a piping connection 128. It is to be understood that theheat exchanger is external of the vehicle retarder.

The rotatable member 102 is provided at the opposite ends with couplings103 and 104 so as to enable it to be connected as part of the propellershaft of a vehicle. The bellows piston actuating means for the frictionplates 2 is designated by the reference numeral 6.

In the interests of clarity, FIG. 11 does not show the valve means andhydraulic circuitry of this invention. Further, since the specifics ofthe mechanical assembly and operation of the vehicle retarder are notpart of this invention, they are not described here although fulldescription may be found by reference to the aforementioned U.S. Pat.No. 3,924,712.

Obviously, modifications can be made to the geometry of the variousembodiments described above without departing from the spirit of theinvention.

We claim:
 1. In a hydraulically-operated friction brake of the kindcomprising a casing for supporting (i) a rotatable member connectable toa vehicle propeller shaft and (ii) a retarding device which includes apack of (a) annular friction plates drivably mounted on the rotatablemember and (b) annular stator plates supported by said casing andinter-leaved with said friction plates, said pack of plates beingrelatively slidable axially of said rotatable member under the action of(c) hydraulically operable actuator means so as to bring said platesinto contact to generate a retarding torque on said rotatable member,and including (d) means for circulating hydraulic liquid through ahydraulic circuit which includes said plates; the improvement comprisingthe provision of valve means operable to control the flow of hydraulicliquid to said plates so that (1) during periods of actuation of saidretarding device at least a major portion of said flow is directed tosaid plates and (2) during periods of non-actuation of said retardingdevice not more than a minor portion of the flow is directed to saidplates, and the volume of said hydraulic liquid contained in said brakeat the maximum operating temperature thereof is less than the totalvolume of said casing including said hydraulic circuit and said actuatormeans, with there always being a liquid-free space within said casing sothat said pack of plates will tend to empty itself of liquid into saidliquid-free space during periods of non-operation of said brake.
 2. Thefriction brake of claim 1, wherein said valve means includes a bodyportion defining a bore, a valve element axially slidable in the boreand having first and second valve members, a first of which is fast onthe valve element and the second being slidable on the valve element andurged away from the first member by means of a spring means, and inletand outlet means for passage of hydraulic liquid into and out of thevalve means disposed so that said major portion of the flow of hydraulicliquid is directed through the bore to the plates when the valve elementis in a first position and so that not more than said minor portion ofthe flow of hydraulic liquid is directed to the plates when the valveelement is in a second position axially displaced relative to the firstposition.
 3. The friction brake of claim 2, wherein said spring meansincludes first and second springs, the second valve member is initiallyurged away from the first valve member by said first spring duringperiods of nonactuation of the retarding device, and said second springis of shorter axial length than said first spring and greatercompressive strength and is provided to compensate for undue variationof pressure on the face of the second valve member remote from the firstvalve member.
 4. The friction brake of claim 2, wherein a third valvemember is provided by a pressure sensitive valve device in an outletfrom the bore communicating with said cooling means for the hydraulicliquid.
 5. The friction brake of claim 2 wherein there is a tubularthird valve member mounted on said valve element in spaced relation tosaid second valve member and in said valve first position closing flowto said plates while permitting hydraulic fluid circulation through saidvalve.
 6. The friction brake of claim 1, including a heat-exchanger influid communication with the valve means and having a liquid outlet fromsaid valve means, and wherein a pressure relief device is associatedwith said outlet to permit the hydraulic liquid to bypass theheat-exchanger, in response to excessive liquid pressures developedtherein during use of the brake.
 7. The friction brake of claim 1,wherein said valve means including a body portion defining a bore, aplurality of passages opening into said bore, said passages defining inaxial sequence:(1) an auxiliary from said pump, (2) a passage to saidhydraulically operable actuator means, (3) a vent passage to saidcasing, (4) a dump passage, (5) a passage to said plates, and (6) apassage from said pump;and a valve element axially slidable in said boreand including first and second valve members carried by said valveelement, said first valve member being uppermost and fixed on said valveelement, said second valve member being slidable on said valve member,spring means urging said second valve member away from said first valvemember; and in said first position said first valve member is at theupper end of said bore closing said auxiliary passage and blockingnormal communication with said vent passage and said passage to saidhydraulically operable actuator means through said bore, and said secondvalve member being positioned between said vent passage and said dumppassage.
 8. The friction brake of claim 7, wherein said spring meansincludes first and second springs, the second valve member is urged awayfrom the first valve member by said first spring during periods ofnonactuation of the retarding device, and said second spring isinitially of shorter axial length than said first spring and greatercompressive strength and is provided to compensate for undue variationof pressure on the face of the second valve member remote from the firstvalve member.
 9. The friction brake of claim 7 wherein there is atubular third valve member mounted on said valve element in spacedrelation to said second valve member and in said valve first positionclosing flow to said plates while permitting hydraulic fluid circulationbetween said passage from said pump through said third valve member tosaid dump passage.
 10. In a hydraulically-operated friction brake of thekind comprisng a casing for supporting (i) a rotatable memberconnectable to a vehicle propeller shaft and (ii) a retarding devicewhich comprises (a) annular friction plates drivably mounted on therotatable member, (b) annular stator plates supported by the casing andinter-leaved with said friction plates, the plates being relativelyslidable axially of said rotatable member under the action of (c) ahydraulically operable actuator means so as to bring the plates intocontact to generate a retarding torque on the rotatable member, andincluding (d) means for circulating hydraulic liquid through a hydrauliccircuit which includes the plates, the improvement comprising theprovision of valve means operable to control the flow of hydraulicliquid to the plates so that (1) during periods of actuation of theretarding device at least a major portion of said flow is directed tothe plates and (2) during periods of non-actuation of the retardingdevice not more than a minor portion of the flow is directed to theplates, cooling means and a pump for circulating said hydraulic liquidthrough said cooling means and through first and second hydrauliccircuits, said first circuit being for the actuation of the retardingdevice and said second circuit being for the circulation of thehydraulic liquid during periods of actuation and non-actuation of theretarding device, said second circuit including said valve means, saidvalve means including a body portion defining a bore, a valve elementaxially slidable in the bore and having first and second valve members,a first of which being fast on the valve element and the second beingslidable on the valve element and urged away from the first member bymeans of a spring means, and inlet and outlet means for passage ofhydraulic liquid into and out of the valve means disposed so that saidmajor portion of the flow of hydraulic liquid is directed through thebore to the plates when the valve element is in a first position and sothat not more than said minor portion of the flow of hydraulic liquid isdirected to the plates when the valve element is in a second positionaxially displaced relative to the first position, said spring meansincluding first and second springs, the second valve member being urgedaway from the first valve member by said first spring during periods ofnon-actuation of the retarding device, and said second spring beinginitially of shorter axial length than said first spring and greatercompressive strength and being provided to compensate for unduevariation of pressure on the face of the second valve member remote fromthe first valve member.